Maybe you should also try the following, at least it solved our problems after trying everything else:
Push down ctrl - alt - del. In the following screen look at the tab "processes"". Stop the proces "Steam.exe".
Try to start Steam again. If it works, log in and go to "settings". Look for the option "start steam when windows starts. Disable it, because steam can only operate with one working copy. That should do the trick.

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